The determinants of which particular combinations of sperm and egg are successful in forming a zygote involve a myriad of molecular, cellular, neurological and whole-organism behavioral attributes. Our past work has focused on the phenomenology of differential success of the sperm in multiply mated female Drosophila, and screens for genes that impact this sperm competition have illuminated many aspects of the process. The first aim of our continuing effort is to determine the role(s) in sperm success of genes expressed by the female in her reproductive tract. To dissect the functional role of these genes in sperm use, we will apply highly targeted RNAi knockdown and assess the impact of loss of expression of these genes in the female reproductive tract. This is readily accomplished with the UAS-GAL4 system, using GAL4 drivers that express in the female reproductive tract. Judicious criteria will be applied to select candidates for knockdown and we are especially interested in the set of genes expressed in females that have neurological function. Knockdowns of female response genes will be tested in a battery of sperm competition tests against a set of 16 inbred sequence-known males and seminal protein nulls to try to quantify male x female interaction as well as testing specific interactions. Our second aim is to quantify genetic differences among males in their ability to influence post-mating transcriptome responses in females, and the roles of those expression changes on sperm competition. Females will be mated with males drawn from the Drosophila Genetic Reference Panel (for which both genome sequence and gene expression data are already available) and reproductive tracts will be dissected and subjected to RNA-sequencing. Analysis will provide a detailed picture of which genes respond transcriptionally to mating, and which genes respond differentially to different male genotypes. These genes will in turn be knocked down in females to see if they play a role in sperm competition. Because the genes whose variation we examine involve basic reproductive functions, our results will expand the foundation for understanding cases of idiopathic human infertility, specifically those in which no sex-specific cause for the infertility can be identified and may thus reflect genetic incompatibility between the partners.